Android UI开发——Fragment间的通信

为了重用Fragment的UI组件,创建的每个Fragment都应该是自包含的、有它自己的布局和行为的模块化组件。一旦你定义了这些可重用的Fragment,你就可以把它们跟一个Activity关联,并把它们跟应用程序的逻辑相连来实现全部的组合式UI。

现实中我们经常想要一个Fragment跟另一个Fragment进行通信,例如,要基于一个用户事件来改变内容。所有的Fragment间的通信都是通过跟关联的Activity来完成的。另外Fragment之间不应该直接通信。也就是说Fragment间不直接通信,通过Activity转一下,按java常规,转一下多是使用Interface实现的。

定义Interface

为了让Fragment跟它的Activity通信,你可以在Fragment类中定义一个接口,并在它所属的Activity中实现该接口。Fragment在它的onAttach()方法执行期间捕获该接口的实现,然后就可以调用接口方法,以便跟Activity通信。

以下是Fragment跟Activity通信的示例:

public class HeadlinesFragment extends ListFragment { 
    OnHeadlineSelectedListener mCallback; 
    // Container Activity must implement this interface 
    public interface OnHeadlineSelectedListener { 
        public void onArticleSelected(int position); 
    } 
    @Override 
    public void onAttach(Activity activity) { 
        super.onAttach(activity); 
        // This makes sure that the container activity has implemented 
        // the callback interface. If not, it throws an exception 
        try { 
            mCallback = (OnHeadlineSelectedListener) activity; 
        } catch (ClassCastException e) { 
            throw new ClassCastException(activity.toString() 
                    + " must implement OnHeadlineSelectedListener"); 
        } 
    } 
    ... 
}

现在,这个Fragment就可以通过调用OnHealdlineSelectedListener接口实例mCallback的onArticleSelected()方法(或其他的接口中的方法)给Activity发送消息。

例如,在Fragment中的下列方法会用户点击列表项时被调用。该Fragment使用回调接口把该事件发送给它的父Activity。

@Override 
   public void onListItemClick(ListView l, View v, int position, long id) { 
       // Send the event to the host activity 
       mCallback.onArticleSelected(position); 
   }

实现Interface

为了从Fragment中接收事件回调,包含Fragment的Activity必须实现Fragment类中定义的接口。

例如,下面Activity实现了上面示例中定义的接口:

public static class MainActivity extends Activity 
        implements HeadlinesFragment.OnHeadlineSelectedListener{ 
    ... 
    public void onArticleSelected(int position) { 
        // The user selected the headline of an article from the HeadlinesFragment 
        // Do something here to display that article 
    } 
}

把消息传递给另一个Fragment

通过使用findFragmentById()方法捕获Fragment实例,宿主Activity可以把消息发送给该Fragment,然后直接调用该Fragment的公共方法。

例如,上面的示例,Activty通过Interface的实现方法,传递数据到另一个Fragment。

public static class MainActivity extends Activity 
        implements HeadlinesFragment.OnHeadlineSelectedListener{ 
    ... 
    public void onArticleSelected(int position) { 
        // The user selected the headline of an article from the HeadlinesFragment 
        // Do something here to display that article 
        ArticleFragment articleFrag = (ArticleFragment) 
                getSupportFragmentManager().findFragmentById(R.id.article_fragment); 
        if (articleFrag != null) { 
            // If article frag is available, we're in two-pane layout... 
            // Call a method in the ArticleFragment to update its content 
            articleFrag.updateArticleView(position); 
        } else { 
            // Otherwise, we're in the one-pane layout and must swap frags... 
            // Create fragment and give it an argument for the selected article 
            ArticleFragment newFragment = new ArticleFragment(); 
            Bundle args = new Bundle(); 
            args.putInt(ArticleFragment.ARG_POSITION, position); 
            newFragment.setArguments(args); 
            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); 
            // Replace whatever is in the fragment_container view with this fragment, 
            // and add the transaction to the back stack so the user can navigate back 
            transaction.replace(R.id.fragment_container, newFragment); 
            transaction.addToBackStack(null); 
            // Commit the transaction 
            transaction.commit(); 
        } 
    } 
}